This monograph is based on a study of the case records of fifty-one patients who had a total of sixty-one attacks of spontaneous pneumothorax resulting from rupture of a vesicle with a valve on the surface of the lung. It includes three cases of such unruptured vesicles found incidentally at necropsy and studied by the author. The case histories are given in detail. There is a comprehensive bibliography. The volume is an exhaustive and authoritative contribution to the subject.